Dom!Santana couldn't be with Dom!Brittney because two Doms can not be together. So she chose sub!Kurt (I guess so Karofsky couldn't pick him or whoever he's matched with wouldn't treat him wrong). They eventually learn to be together and have a child but then Santana dies. Kurt has a hard time coping, so he does these tricks to get himself through the day (like imaging Santana telling him to go to the grocery store; etc.) One day he's in a rush(I'm pretty sure its to get his kids medicine. They ran out) and he's running with a stroller and he trips and the stroller goes flying into traffic. That's where Blaine enters. Blaine sees the rolling stroller from afar and jumps out of his (limo?) and stops it. Kurt is pretty mush hyperventilating. Blaine has somebody take the baby to the hospital or Kurt's house. He realizes Kurt's going through the sickness really bad so Blaine has to Dom him. They enter a temporary contract, but Blaine believes that subs don't really need a Dom. He tries to get Kurt to make decisions but it makes him more sick.
It starts off Santana/Kurt but it is definitely Klaine.
Thanks for taking the time to read this:)